Fix inlining of references to external files

[dinwwwh]

Summary

This PR fixes a bug in the reference inlining logic where references to external files were not being properly inlined when they were themselves targets of internal references.

Key Changes

• Fixed isInlinable logic in shared.ts: Changed the condition to check if a reference resolves to an inlinable target (record or boolean) AND has a valid alias end, rather than checking the alias end first. This ensures that references to external files (which have no alias chain) are correctly identified as inlinable when they are direct targets.

• Added test coverage for external file references:

  • Added test in v3.1-to-v3.0.test.ts to verify path items referencing external files are inlined correctly
  • Added test in v3.1-to-v3.0.test.ts to verify schema $defs entries referencing external files are inlined correctly
  • Added test in shared.test.ts to verify references whose targets are themselves external references are properly inlined
  • Updated test in v3.2-to-v3.1.test.ts to include external file reference scenarios

Implementation Details

The core issue was in the isInlinable function which was using aliasEnd(ref) to determine if a reference could be inlined. However, aliasEnd returns undefined for references that don't have an alias chain (like direct external file references), causing them to be incorrectly marked as non-inlinable. The fix reorders the logic to first resolve the reference and check if it's a valid target, then verify it has a proper alias chain, allowing external file references to be inlined when appropriate.

https://claude.ai/code/session_011uRMv7RDjXFSBo71sjcefJ

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[pullfrog]

✅ No new issues found.

Reviewed changes

• isInlinable in downgrader/src/shared.ts: now checks the ref's immediate target (resolveRef(ref) is a record or boolean) plus that aliasEnd(ref) terminates, instead of resolving the chain's terminal target. Refs whose target is itself a $ref to an external file are now inlinable, so they no longer survive as dangling references inside removed parts.
• Regression tests: shared.test.ts gains a removed-alias-to-external case; v3.1-to-v3.0.test.ts gains components.pathItems and $defs entries pointing at external files; the v3.2-to-v3.1.test.ts parameter-shift test now inserts an external entry so Shifted rebases.

I confirmed the four added/modified tests fail against the previous isInlinable condition and that the full suite (487 tests) passes at 21dc45e. Cycles remain non-inlinable because aliasEnd returns undefined for them, and direct external refs still stay as written. No README change needed — line 139 already documents "following the reference chain until it leaves the removed part".

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